Industrial processes often require measuring the level of liquid or other material in a tank. Many technologies are used for level measurement. With contact level measurement some part of the system, such as a probe, must contact the material being measured. With non-contact level measurement the level is measured without contacting the material to be measured. One example is non-contact ultrasound, which uses high-frequency sonic waves to detect level. Another example is use of high-frequency or microwave RF energy. Microwave measurement for level generally uses either pulsed or frequency modulated continuous wave (FMCW) signals to make product level measurements. This method is often referred to as through air radar. Through air radar has the advantage that it is non-contact and relatively insensitive to measurement errors from varying process pressure and temperature. Known radar process control instruments operate at frequency bands of approximately 6 Ghz or 24 Ghz.
A through air radar measurement instrument must convert a high frequency electrical signal to an electromagnetic wave. An oscillator is used to generate the high frequency signal. An antenna, such as a waveguide or horn, is operatively associated with the oscillator. A microwave frequency (26 GHZ, for example) radiation beam is propagated downward from the antenna, and reflected off the surface of the material being measured to the antenna where the signal is received. The product level is calculated from the total time of propagation of the signal.
A difficulty can be encountered when a metal object is located in or around the radiated electromagnetic field. A reflection from a metal object can cause a false target situation, in which the system evaluates the product to be at a level indicated by the reflected signal from the object and not from the actual product. Typical false target objects in tanks are mixers, nozzles, ladders and tank walls. It is well known that the electromagnetic field pattern radiated by a waveguide and/or antenna structure has a characteristic commonly referred to as “polarization”. This term refers to the alignment or orientation of the electric and magnetic field components of the radiated wave. A common polarization characteristic is called “linear polarization”, in which the radiated electric and magnetic fields are oriented at ninety degrees with respect to each other. Linear polarization is common in many RF systems as most simple antenna structures are known to radiate in this manner. Other forms of polarization exist but they are typically the result of more complex and expensive antenna/circuit structures.
A characteristic of the linearly-polarized radar signals is that orientation of the beam, i.e., orientation of the electric and magnetic fields, will produce a different radar reflection if the object in the beam is not “fully illuminated” by the beam (such as from a large, flat surface). Typical false target objects in a tank are small and asymmetric in the beam (unlike the large, flat liquid surface) and, therefore, orientation of the beam can have a large effect on the susceptibility of the radar to “see” and, therefore be disturbed, by, these smaller unwanted objects.
It is advantageous, therefore, for the radar transmitter to have a convenient form of varying, or rotating, the orientation (“polarization”) of the radar beam to minimize the effect of unwanted objects in the radar beam in the actual installation.
An alternative is disclosed in Janitch et al. U.S. Pat. No. 7,106,248, assigned to the assignee of the present application. The instrument described therein utilized a specialized coupling between the housing and antenna to allow independent rotation to achieve optimum orientation of the radar signal. This design requires numerous machined metal parts, O-rings and specially formed electrical components to facilitate independent rotation of the transmission signal relative to the electronics housing.
The present invention is directed to overcoming one or more of the problems discussed above, in a novel and simple manner.